Various techniques may be used to evaluate geological formations. For example, measurements may be made using tools located within a borehole such as in support of geophysical and petrophysical exploration or resource extraction. In one approach, an apparent resistivity (or conductivity) of a formation may be evaluated such as by injecting a current from a location within the borehole into a portion of the formation, and conductively measuring a resulting voltage induced by the current. Such resistivity information may provide a general indication of formation composition or geometry, including providing indicia of invasion or hydrocarbon presence.
Early examples of this type of borehole resistivity (i.e., “galvanic”) measurement tools generally provided virtually no control over a radial depth of resistivity investigation into a formation. Later examples included one or more “guard” electrodes configured to provide an equal potential (or “equipotential”) region in a medium nearby an electrode array, thus forcing a larger proportion of the injected current into the formation than earlier tools. Such a configuration is generally referred to as a “laterolog” or “guard log” tool.
Further development of laterolog tools included providing an array of current and monitor electrodes, such as to provide resistivity logging for a variety of relatively shallower or relatively deeper radial depths of investigation. In an array laterolog, focusing of an injected current may be established using hardware or software techniques, or a combination of both hardware and software techniques.
Despite these advances, certain measurement scenarios may still be problematic for array laterolog measurements. For example, when a formation resistivity is much larger than a resistivity of mud located in the borehole, the measured voltage differences between some of the monitor electrodes may be very small, or even below a noise floor of the system. Measurements using such affected monitor electrodes may therefore be impractical because of error induced by noise or interference in the system. Also, an apparent formation resistivity determined using such confounded measurements may be inaccurate, particularly after software-based focusing of information obtained from the affected monitor electrodes.
In one approach, attempts to manage such errors have included mechanical and electrical solutions that attempt to reduce coupling between electrodes or attempt to reduce a system thermal noise. In other approaches, attempts have been made to reduce noise effects by averaging, reduction of tool dimensions, or by increasing the emitted current in order to increase the monitor electrode voltages. An alternative approach is presented below.